This application is in the area of methods for the adjustment of pH in textile processes.
Fibers are processed in a number of ways before incorporation into textile products such as fabric and carpet. In a typical procedure in the case of nylon, nylon chip is first extruded and then converted into either continuous filament or staple fiber. The filament or staple fiber is then spun into yarn, and twisted and heat set as desired. The undyed, untreated yarn can be dyed and finished before or after fabrication into a final product.
The dying procedure can occur by batch or continuous operation. Again, in the case of nylon, in either operation, the dye bath typically includes an acid dye (i.e., a dye that adheres to the nylon under acidic conditions), a chelating agent, an acid dye leveler (an agent that promotes the even absorption of dye by the yarn), a wetting agent, and a pH adjuster (to insure acidic pH). The dye bath for continuous processing is typically maintained at a pH of between 3 and 6.5, and more typically, between 4 and 5. The dye bath for exhaust application is typically maintained at a pH of between 6 and 7, and more typically at pH 6.5. The dyed yarn is then taken through a fixing step (usually moist heat), and rinsed.
The dyed yarn is finished by the application of desired finishing chemicals to the yarn, including stain blocking agents, fluorochemicals, fabric softeners, stabilizers, UV absorbers, optical brighteners, sewing assist agents, antistatic agents, waterproofing agents, durable resins, starches, and sizes. Some of these agents adhere more effectively when applied under acidic conditions. For example, common commercial stain blocking chemicals such as BASF Stain Protector and DuPont Stain resist chemicals are usually more effective when applied at a pH of between 1.9 and 2.5. Fluorochemical polymeric finishes are usually applied at a pH of between 4 and 5. Coapplication of a stain blocking agent and a fluorochemical in a foam is often carried out at a pH of 3-5.
Yarns and fibers made from materials other than nylon can also be dyed or treated under acidic conditions. Wool and silk are typically acid dyed. Polyester is often disperse dyed with a solution that may have an acidic pH.
Traditional chemicals used for pH adjustment in textile processing include sulfamic acid, formic acid, sulfuric acid, phosphoric acid and combinations thereof. These pH adjusters can exhibit one or a combination of low efficiency (i.e., require large amounts of chemical for the desired effect), high cost, difficulty in handling, hazards in handling, corrosiveness, or high acidity of the resulting effluent stream.
One of the most commonly used pH adjustment system used in carpet processing is sulfamic acid ((HO)S(O).sub.2 NH.sub.2). Lower pHs can be attained with sulfamic acid than with comparable amounts of formic acid. Sulfuric and phosphoric acids will lower pH more efficiently than either sulfamic or formic acids. Corrosivity tests performed on carbon steel and stainless steel coupons show that all of these acids can be harmful to metal equipment.
Sulfamic acid is a solid that can be dissolved in water up to a concentration of about 15% at room temperature. Dissolving the acid is cumbersome and represents an additional step in the textile manufacturing processes while creating another quality control step.
Since sulfamic acid is only soluble in water to an extent of about 15% by weight at room temperature, and is usually used in a 13% solids solution, it is not a very efficient pH adjustment system. Approximately 6-grams/liter of 13% sulfamic acid solution is required to lower the pH of a typical stain resist application bath to a pH of 2, depending on the concentration and type of stain blocking chemical used.
While sulfamic acid is weaker than sulfuric acid, sulfamic acid powder can cause burns and thus requires special procedures for handling. Sulfamic acid solution is also quite corrosive, and thus has a damaging effect on machinery, structural supports and concrete. Sulfamic acid solution even corrodes 316 stainless steel coupons and carbon steel.
The use of sulfamic acid increases the cost of textile processes because of the time and expense incurred to dissolve the powder, and the cost involved to repair and maintain corroded equipment. Further, as a significant amount of the acid used is carried into the effluent stream, the effluent must be neutralized to a significant extent with caustic prior to discharge to the water treatment facility, further adding to the cost of the textile process.
The object of the present invention is to provide a method for pH adjustment in textile processing that includes the utilization of a chemical that is more convenient, less corrosive, more efficient and less harmful to the environment than the currently used processes.